Periodate oxidation of saccharides. III.* Comparison of the methods for determining the consumption

of sodium periodate and the amount of formic acid formed

K. BABOR, V. KALÁČ, and K. TIHLÁRIK

Institute of Chemistry, Slovak Academy of Sciences, 809 33 Bratislava

Received 27 September 1972

Different methods for determining the oxidizing agent consumption during the periodate oxidation of glucose have been compared. I t has been found that the methods requiring alkaline medium are not suitable for the period­ate determination. Using these methods, a further periodate consumption may occur during the determination.

On the basis of unexpected differences in the determination of free and bound formic acid formed by periodate oxidation of glycerol and mannitol a possible course of the periodate oxidation of 1,2-diols is discussed.

When, investigating the periodate oxidation of glycols and saccharides we have ob­served several times a disagreement between the oxidizing agent consumption and the determined amount of formic acid formed by oxidation. I t has also been noted that the results of determination are dependent on the application of particular methods.

Also other authors, e.g. Hughes and Nevelí [1] found in the periodate oxidation of glucose that after the periodate consumption the amount of formic acid formed during the oxidation was smaller than expected. The authors used three methods for determin­ing the decrease of the periodate ion: 1. thiosulfate method in acidic medium, 2. arsenite in alkaline medium according to Fleury —Lange, and 3. the latter one modified by these authors. They observed that after one-hour oxidation of glucose, the periodate consump­tion determined by the original Fleury — Lange method was almost о moles/mole glucose; however, the result of determination of periodate consumption by thiosulfate in acidic medium reached the value of about 3 moles/mole glucose. The value obtained by the third method was intermediate between the above two.

For the determination of the formic acid formed the authors [1] used the direct titra­tion with sodium hydroxide using Phenolphthalein as indicator, which, in agreement with our experiences gave the value close to total formic acid, i.e. including the portion bound as formy 1 derivative. This method proved the formation of 3 moles of formic acid per 1 mole of glucose after one-hour oxidation of glucose; this corresponds to the periodate consumption determined in acidic medium.

Fedorohko et al. [2, 3] observed during the periodate oxidation of glyceraldehyde that upon 15 minutes' oxidation the periodate consumption determined by the method of Fleury —Lange is theoretical, whereas the formic acid determined alkalimetrically represents only one quarter of the theory.

* For Part I I see Ref. [G].

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PERIODATE OXIDATION OF SACCHARIDES. I l l

Pratt it al. [4] no ted t h a t in t he oxidat ion of sedoheptulosane t he theoret ical per iodate consumpt ion was a t t a ined after one hour b u t t h e formic acid being formed reached the expected value only after 12 days . The au thors have no t , however, described the me thods of de terminat ion t h e y used.

Different me thods of t h e formic acid de te rmina t ion have been compared a n d t h e iodometric m e t h o d of de te rmina t ion bo th of free formic acid [5] a n d of formate por t ion [6] has been worked u p . This m e t h o d provides accura te a n d reproducible resul ts . The principle of t h e free formic acid de te rmina t ion lies in t h e fact t h a t after des t roying the excess of per iodate wi th e thylene glycol there are added to t he solution potass ium iodide and sodium thiosulfate, t he excess of which is back- t i t r a t ed wi th iodine. T h e to t a l formic acid de terminat ion , including t h e formate port ion, is based on t h e fact t h a t upon addi­t ion of sodium thiosulfate dur ing four-hour s tanding , hydrolysis of t he formate port ion of formic acid t akes place in t he solution by shifting t he react ion equil ibr ium. Hydrolys is t h u s proceeds in neu t ra l med ium which el iminates t h e errors of alkal imetr ic m e t h o d of t he formic acid de terminat ion . These errors migh t be due to alkali consumpt ion by a ldehyde groups of oxidat ion p roduc t s . The appl icat ion of b iamperomet r ic indicat ion el iminates t h e error of t he original iodometric method . I n th is m e t h o d s ta rch or amy-lose were used as indicators , these binding a considerable a m o u n t of iodine and t h u s influencing the accuracy of t h e formic acid de te rmina t ion .

B y means of t h u s modified me thod we decided to invest igate t h e per iodate oxidat ion of several model substances , including glucose oxidized in [1], compare t he results obta ined by different me thods , a n d explain discrepancies in individual de te rmina t ions , bes ides t he de te rmina t ion of formic acid, we compared the me thods of tho per iodate de te rmina­t ion in acidic and alkaline media with the spec t ropho tome t ry me thod .

Resu l t s a n d discussion

A. Periodate oxidation of glucose

The results of per iodate oxidat ion of glucose are listed in Table 1. W e m a y conclude from tho results obta ined t h a t for t h e de te rmina t ion of per iodate

concentrat ion t he me thods requir ing alkalinization of t h e m e d i u m before proper de­te rmina t ion (methods 2 and Z in Exper imenta l ) are no t su i tab le : in case a p roduc t , which is a formy 1 der ivat ive , has been formed b y oxidat ion, a further consumpt ion of

Table 1

Per ioda te oxidat ion of D-glucose

Determined moles/mole glucose

Free formic acid Total formic acid Sodium periodate consumption in acidic medium (methods 1 and 4) Sodium periodate consumption in alkaline medium (methods 2 and 3)

Duration of oxidation

1 hour

1.93 2.97

2.98

4.11

2 hours

2.24 3.15

3.52

5.00

Chem. zvesti 27 (ó) 676-C80 (1973) a 7 7

K. BABOR, V. KALÄO, K. TIHLÁRIK

periodate occurs during determination in alkaline inedium. The only convenient methods for determining the instant periodate concentration are considered to be the method 1 (i.e. in acidic medium) or the method 4 (the spectrophotometry method).

Bu'ust and co-workers [7, t] proved the following stages of the mechanism of periodate oxidation of 1,2-diols: formation of monoester from periodate ion and hydroxyl group of diol. the ring closure with adjacent hydroxyl group under the formation of a cyclic ester, and the final stage — the ester decomposition into reaction products; the indi­vidual stages of reaction are acid-base catalyzed (Scheme 1).

R2C-OH

R2C—OH

»2«>S . R2C-0—10uH-

R2C-OH

Scheme 1

R2C-0

- 1 >5 R2C-0

-— 2R2C=0 • IOj

From this point of view the results of glucose oxidation we obtained may be interpreted as follows. Glucose consumed during the first hour of oxidation (Table 1) 3 moles of periodate mostly for the oxidative cleavage of carbon —carbon bonds C-3 — C-2. C-:.> — C-3, and C-3 —0-4 under the formation of two formic acids and a formyl derivative of gly-ccraklehyde (Scheme 2). The formyl derivatives are relatively stable in slightly acidic medium and only slowly hydrolyze [0]. Thus the formed formyl derivative of glyceralde-hyde does not oxidize any more in the solution with the excess of periodate ions and we assume onlv irs esterification.

CH2-0H . CH2-0-lO^H' 3H2l°5 ^ 2HC00H CH-O-CHO ^ ^ - CH-O-CHO

I I CH-OH CH-O—IO.H' I I

OH OH

Scheme 2

When determining the amount of periodate by means of titration in acidic medium or spectrophotometrically (methods 1 and 4) the portion of periodate bound as ester is also determined. When using methods 2 and 3 (alkaline medium) for the determina­tion, on addition of alkaline buffer or saturated sodium hydrogen carbonate solution the prompt hydrolysis of formyl derivative takes place. Since the periodate ion is more prone to solvation in alkaline medium [9], the more rapid formation of cyclic ester appears, this being base catalyzed [7] and subsequent decomposition into oxidation products then follows. For this reason, the periodate consumption determined under the given conditions is at the same time of oxidation higher than in the determination by the methods in acidic medium.

B. Periodate oxidation of glycerol and mannitol

We have found in the periodate oxidation of glycerol and mannitol that 2 and 5 moles of periodate respectively, per 1 mole of the corresponding glycol have been consumed after one-hour oxidation; each of the four methods used leads to the theoretical value. The deter­mination of the total formic acid (method 6) gives also the theoretical value. However, in

678 Chem. zvesil 27 (5) 676-680 (197?)

PERIODATE OXIDATION OF SACCHARIDES. I l l

the case of glycerol the determined amount of free formic acid is only 82% of its total (method 5) and even after 120-hour standing it is only 85 °7 ; in the case of mannitol it makes 78 and 81%, respectively.

In the course of the periodate oxidation of the substances mentioned above, the for-myl derivative, which, in the case of glucose, hinders further oxidation in slightly acidic medium, is not formed. Therefore the oxidation of glycerol and mannitol proceeds conti­nuously and the decrease of periodate determined by different methods is identical. The difference between the determined free and total formic acid not bound as formyl ester remains unexplained. The formation of acylals by the reaction of formic acid with carbonyl groups of the oxidation product or the formation of acylals as intermediate (according to Scheme 3), which is in the given case relatively stable, may be assumed.

OH I

RC-OH

R2C-OH

-OH

н2ю-5 Rc-o-io^H-

R2C-OH

OH I

RC-O

)°3 R0C-0

OH

RC—OH

)° RpC—OH

Ю5 .RCOOH * R0CO

scheme 3

In this reaction mechanism new bond between free oxygen of periodate and carbon atoms of diol would be formed during the oxidation. This may be confirmed by the fact that in the cases when this oxygen is bound in another ester form, oxidation does not take place. This is the case with the periodate oxidation of tridentates [10]. Further, it has been proved on the basis of isotopic reactions with periodate containing isotopes 1 8 0 that oxygen in the oxidation products comes from periodate [11].

Experimental

For biamperometric titrations a Multiflex (Lange, Germany) galvanometer was used; sensitivity 4 x 10 - 9 A/cm, connected with the circuit in the visual manner. The platinum electrodes were supplied with a voltage of 30 mV. Absorbances were measured with an H 700.307 (Hilger, England) spectrophotometer.

Pure glucose and mannitol (Lachema, Czechoslovakia), crystallized from dilute ethanol and glycerol, anal, grade (Strem, Poland) were used. Other chemicals were anal, grade.

Methods Periodate oxidation

Periodate oxidations of glycerol, D-glucose, and D-mannitol were performed in volu­metric flasks in the dark at laboratory temperature. The stock sodium periodate solu­tion (50 mg NaI0 4/ml) was added to samples in such an amount so as to obtain 20 — 50% excess of periodate and its resulting solution of 0.02 —0.05 м. After making up to the mark with distilled water, samples were withdrawn in appropriate time intervals to determine the concentrations of sodium periodate and formic acid formed.

Determination of sodium periodate

I. Determination of periodate in acidic medium

I. A sample (content about 1 mg of N a I 0 4 ) was pipetted into a beaker with water (about 30 ml), then 0.1 N - H 2 S 0 4 (5 ml) and 0.4 м-KI (5 ml) were added. On stirring,

Chem. zvesti 27 (5) 676-680 (1973) 6 7 9

К. ВАВОВ, V. KALÁC, К. TIHLÁRIK

0.01 N-Na2S203 (10 ml) was added and back-titrated with the 0.01 N-I2 solution by bi-amperometric indication (consumption Na2S203 I).

II. A sample of solution was pipetted as previously, ethylene glycol (0.5 ml) was added and allowed to stand for 15 minutes. Then 0.1 N - H 2 S 0 4 (5 ml) and 0.4 M - K I (5ml) were added and the reaction proceeded in the same way as before (consumption "Na2S,03 / / ) •

Calculation: 1.07 X (consumption / — consumption II) = mg N a I 0 4 in a sample.

2. Determination of periodate in alkaline medium

A sample (content about 1 mg of NalO,) was pipetted as in the above-mentioned cases, saturated NaHC03 solution (1ml), 0.4 м-KI (5 ml), and 0.01 N-Na 2S 20 3 (10 ml) were added. After 15 minutes it was back-titrated with the 0.01 N-I2 solution by bi-amperomctric indication.

3. Determination of periodate according to Fleury — Lange [12]

To a sample (content about 1 mg of NaI04) saturated N a H C 0 3 solution (1 ml). 0.01 N-Na3As03 (10 ml), and 0.4 м-KI (5 ml) were added. After 15 minutes it was back--titrated with the 0.01 N-I2 solution by biamperometric indication.

4. Determination of periodate épectroj.hotomttrically [13]

The periodate consumption in samples was determined by measuring the absorb ance decrease at 223 nm.

Determination of formic acid D ztermination of free formic acid

A sample (content about 1 — 5 mg of HCOOH) was pipetted into a beaker with water, ethylene glycol (0.5 ml) was added and let stand for 15 minutes. Then 0.4 м-KI (5 ml), 0.01 x-Na 2S 20 : } (10 ml) were added and back-titrated immediately w:ith the 0.01 x-I 2

solution by biamperometric indication.

Determination of total formic-acid The procedure was the same as in the free formic acid determination; however, on

adding 0.01 N-Xa2S20 : ) (10 ml) and stirring, the back-titration with the 0.01 x-I« was carried out after standing for 4 hours.

References

1. Hughes, G. and Nevelí, Т. P., Trans. Faraday Soc. 44, 941 (1948). 2. Fedoroňko, M., Fuleová, E., and Danieliszyn, W., Chem. Zvesti 27, 67 (1973). 3. Danieliszyn, W., Thesis. Slovak Technical University, Bratislava, 1965. 4. Pratt, J . W„ Richtmyer, N. K., and Hudson, C. S., J. Amer. Chem. Soc. 74. 2200

(1952). 5. Babor. K., Kaláč, V., and Tihlárik, K., Chem. Zvesti 18, 913 (1964). 6. Babor, K., Kaláč, V., and Tihlárik, K., Chem. Zvesti 20, 595 (1966). 7. Buist, G. J . and Bunton, C. A., J. Chem. Soc. (B) 1971, 2117. 8. Buist, G. J . and Bunton, C. A., J. Chem. Soc. {B) 1971, 2128. 9. Jahr, K. F. and Gegner, E., Angew. Chem. 79, 690 (1967).

10. Nevelí, T. P., Chem. Ind. (London) 1959, 567. 11. Bunton, C. A. and Shiner, V. J., J. Chem. Soc. 1960, 1593. 12. Fleury, P. F. and Lange, J., J. Pharm. Chem. 17, 107 (1933). 13. Aspinal, G. O. and Ferrier, R. J., Chem. Ind. (London) 1957, 1216.

Translated by A. Luk«áčová

680 Chem. zvesti 27 (5) 676-680 (1973)